Liquid Crystal Displays (LCDs) are used in a variety of electronic devices like mobile phones, handhelds, pocket computers or the like in order to display images to a user.
Such LCD displays are often configured to be used in two different modes: In a first mode, the LCD is illuminated by means of a backlight, and in a second mode, said backlight is switched off and only ambient light is used to illuminate the LCD. For instance, said second mode is used in a standby mode of a mobile phone in order to reduce power consumption, and when a user activates the mobile phone, then the backlight is switched on and the LCD is driven in the first state in order to display brilliant color images.
Such a two-mode LCD may comprise a separate light reflective region area 120 and a separate light transmissive area 110 per each pixel, as depicted in FIG. 1a. A backlight 150 is deposited on the back side of the light transmissive area 110 so that light emitted from the backlight 150 can pass through the transmissive region 110, the liquid crystal layer 130 and the transparent upper electrode 140. The reflective region 120 may comprise a reflective layer 121 so that ambient light entering from the observer side of the display may go through the LCD (i.e. the transparent upper electrode 140 and the liquid crystal layer 130) in order to be reflected at a reflective layer 121 of the light reflective area 120 and then the reflected light goes through the LCD again, as depicted on the right hand side of FIG. 1a. As the material of the transparent upper electrode 140, for example, ITO (Indium Tin Oxide) can be used.
Both the light transmissive area 110 and the light reflective area 120 may comprise a controllable electrode 112,122 in order to control the light transmittance of the adjacent part of the liquid crystal layer 130. These controllable electrodes 112,122 may be controlled by a thin film transistor 123.
FIG. 1b depicts a cross-sectional view of three adjacent pixels of an LCD, wherein said cross-sectional view shows a cut beneath the liquid crystal layer 130 depicted in FIG. 1a, and wherein the first light transmissive area 110 and the first light reflective area 120 are associated with a first pixel, the second light transmissive area 110′ and the second light reflective area 120′ are associated with a second pixel, and third light transmissive area 110″ and the third light reflective area 120″ are associated with a third pixel, and wherein the light transmissive areas 110,110′,110″. Each of the thin film transistors 123,123′,123″ is configured to control the respective electrodes 112,122 112′, 122′ 112″,122″ of the associated light transmissive/reflective areas.
For instance, the first light transmissive area 110 may be associated with a green pixel by means of a green color filter (not depicted in FIG. 1b), the second light transmissive area 110′ may be associated with a blue pixel by means of a blue color filter (not depicted in FIG. 1b), and the third light transmissive area 110″ may be associated with a red pixel by means of a red color filter (not depicted in FIG. 1b). Furthermore, the light reflective areas 120,120′,120″ may represent black and white (B&W) reflective regions 120,120′,120″, i.e. no color filters are associated with these B&W reflective regions 120,120′,120″, and the B&W reflective areas 120,120′,120″ reflectivity is controlled by means of the electrodes 122,122′,122″ so that the reflectivity correlates with an image content.
These B&W reflective areas 120,120′,120″ depicted in FIG. 1b are very good for standby mode visibility in case the display is driven in the second mode, i.e. when the backlight 150 is switched off.
Unfortunately, in case the display is driven in the first mode, i.e. it is illuminated with light emitted from the backlight 150 in order to pass the light transmissive regions 110,110′,110″ associated with the different colors, then said B&W reflective areas 120,120′,120″ may cause a decreased image quality under bright environmental illumination, because the black and white light reflected from said B&W reflective areas 120,120′,120″ washes out the colors generated by the backlight and the light transmissive areas 110,110′,110″.